FMF - Friends of Minerals Forum, discussion and message board

[Matt_Zukowski]

How is OH, F, and Cl distributed among apatites? In calcite - dolomite - magnesite, the occurrence in nature is close to pure CaCO3 - CaMg(CO3)2 - MgCO3, with little substitution between these species. This is truly a trimodal distribution along the Ca-Mg dimension. Is the same true along the OH-F-Cl trimension (word?) in the apatite subgroup?

When i first had mineralogy, there was a mineral called "apatite" with the formula Ca5(PO4)3(OH,F,Cl). Now even mindat doesn't show a chemical formula for "apatite" but does show a formula for fluorapatite, chlorapatite, etc.

I spent some time googling IMA+apatite and found all sorts of sorts of complications concerning minerals that have a similar structure to what i would have called apatite. But I couldn't find anything about the substitutabilty (word?) of OH, F, and Cl in the calcium phosphate species.

What separates [apatite with OH-, F-, and Cl- varieties] from [distinct hydroxl-, fluor-, and chlor- species]?

I want my "apatite" back.

Sorry if this post is but another rant on IMA nomenclature.

[Riccardo Modanesi]

Ok listen: unless there is a specific name for a mineral (i.e. "almandine", "grossular" etc. for garnet group or "elbaite", "liddicoatite", "schorlite" for tourmaline group) I always call it simply by its old name. I don't ask myself whether it is a fluorapatite an hydroxylapatite etc. I always call it "apatite". And for example for garnet group, I call a garnet "almandine" not "garnet-Fe-Al"! If it is almandine of course, don't misunderstand me!
Gretings from Italy by Riccardo.

[Jordi Fabre]

Recommended reading:

- Changing Mineral Nomenclature again -> https://www.mineral-forum.com/message-board/viewtopic.php?t=253

- IMA or not IMA? -> https://www.mineral-forum.com/message-board/viewtopic.php?t=95

[GneissWare]

For many purposes, and unless you have access to analytical equipment, the old name is the best you are going to be able to use.

I remember spending an hour or two doing online research on a hornblende specimen. I knew it was hornblende, as it met all the criteria learned in mineralogy lab. But, alas, hornblende no longer exists in the IMA world -- so was it magnesiohornblende or ferrohornblende? Who knows! Even the literature on the locality suggested both were present. I have no intention of having it analyzed, so it is still hornblende to me.

In fact, when I put together the mineral table for my mineral catalog software, I was careful to put in all the IMA approved minerals, such as fluoroapatite (or is now, once again, apatite-F -- they've changed it back and forth). I also added a field called "IMA approved" because I found I couldn't always catalog to IMA names. Sometimes you need to call a mineral Garnet, because that's as close as you can get without analysis.

Man, its tough talking about IMA names without it starting to sound like a rant ;=))

[lluis]

Hi, Matt, Riccardo, GneissWare, Jordi

Welcome to the party!

When I started the discussion about IMA in spanish forum, I felt like the only one, the heretic one (I love ot be this! Heretikos means, as far as I read - I was teached latin, but for my sorrow, not greek - the right to choose!)...

Seems that maybe there is a silent majority that from time to time dares to express his discomfort about splitting hairs, odd decissions (rheniite, wiluite, andyroberstite, Allanite-Ce, pezzotatite are always bewondering me; not as much as Heyite-calderonite (same species, two names! Yeahhhh!) ) and so.

Well, would not be "quis qustodet ipsos custodes", but something like "who would control the reasons of names givers?"

With best wishes

Lluís

[Gerhard Niklasch]

Hello Matt,

You might wish to try refining your search terms.

While I wasn't able to immediately locate something that would directly answer your question about -- what is the word, miscibility? --, I did find a few interesting things by throwing
Apatite F Cl solid solution substitution
at the big G. That was just one ad hoc attempt of mine, further refinements might unearth more!

One thing I learned on the quick was that the three anions would occupy somewhat different positions in the crystal lattice. Cl- is a bit too large to fit at where the F- normally goes.

May the Google-fu be with you!

Enjoy,
Gerhard

[Carles Millan]

If you are not sure about its chemical composition, I think there is nothing wrong with saying 'that is an apatite', no matter the series is trimodal or continuous. The same for tourmaline, garnet, and even wolframite. The vast majority of mineral collectors can't afford to have a complete and precise analysis of their specimens done. Who cares if a good tourmaline is elbaite or liddicoatite or schorl? Or a mix of them? And perhaps there is no such thing as a pure hübnerite or a pure ferberite. Can then we really say there exist pure fluorapatites and pure chlorapatites? Even if the F and Cl ionic radii (or radiuses) are somewhat different in size. The crystal lattice might be slightly distorted, something very usual.

[Matt_Zukowski]

Thanks to everyone for their solidarity. And thanks to Gerhard for encouraging me to broaden my search parameters. I finally found a paper that explains the reasoning behind apatite nomenclature as of March 2010:
https://pubsites.uws.edu.au/ima-cnmnc/Pasero%20et%20al%202010%20apatite%20nomenclature.pdf
(link normalized by FMF)

Unfortunately, they do not specify why the IMA is a "splitter" rather than a "lumper" with regard to the Ca5(PO4)3-apatites. They say only that, "The birth of the three distinct names to denote the F-, OH-, and Cl dominant variants, and their distinction with respect to the original ‘‘apatite’’ sensu lato is uncertain, but is generally ascribed to Damour (1856) for ‘‘hydroxylapatite’’ and Rammelsberg (1860) for ‘‘fluorapatite’’ and ‘‘chlorapatite’’." I could not find Damour (1856), and the version of Rammelsberg (1860) I could find is scanned and thus not amenable to machine translation into english. I remain uncertain as to why the IMA is a splitter and not a lumper.

In one of the forum posts pointed out above by Jordi, John White makes reference to his letter in Rocks & Minerals concerning his preference for using suffixes as in apatite-CaF rather than prefixes as in fluorapatite. The paper i found states:
"Recently, in the context of a revision of the mineralogical nomenclature initiated by then chairman of the IMA Commission on New Minerals, Nomenclature and Classification E.A.J. Burke, and aimed at adopting, as far as possible, modified Levinson suffixes instead of adjectival prefixes such as ‘‘fluor-’’, ‘‘chlor-’’, and ‘‘hydroxyl-’’, the above minerals were renamed apatite-(CaF), apatite-(CaCl), and apatite-(CaOH), respectively (Burke, 2008). One of the rationales for that change was the benefit of having the names of these minerals appear consecutively in alphabetical listings and databases.

The changes introduced by Burke (2008) to the nomenclature of these and other minerals with the apatite structure, e.g. strontium-apatite and ‘‘ellestadites’’, did not fully consider the structural complexities of these minerals. The apatite structure type is flexible enough to allow a wide degree of substitutions among cations (cf. White & Dong, 2003; White et al., 2005), and this fact, coupled with the possible lowering of symmetry related to ordering of cations and anions, dramatically increases the number of end-members which are potentially eligible as individual mineral species.

The recently approved nomenclature scheme of Burke (2008) could logically be extended to the renaming of other apatite group minerals, changing, e.g., pyromorphite into apatite-(PbCl), or alforsite into apatite-(BaCl). And one could be tempted to include also the various tetrahedral cations (P, As, or V), into the extended suffix [e.g., apatite-(PbAsCl) instead of mimetite, apatite-(CaAsOH) instead of johnbaumite]. The result would be mineral names that are more similar to chemical formulae. The limit of such an approach would be to adopt chemical formulae throughout, instead of mineral names. However, a nomenclature based extensively upon modified Levinson-style suffixes is likely to be rejected by the mineralogical community, since many would argue that multiple suffixes are difficult to be read, almost impossible to be spoken, not immediately self-explanatory, and unpleasant to the eye. Furthermore, such naming replaces many traditional names given to honour worthy individuals."

The paper goes on with more on this topic, but I thought i'd post this here for John and others that may be interested.

[Matt_Zukowski]

As to substitutability, I found two papers that seem to me to imply that there is quite a bit of substitution of F for Cl for OH in nature, although there is quite a bit of jockeying around within the structure to make room for the different sized ions. The papers are:
https://rruff.geo.arizona.edu/doclib/am/vol74/AM74_870.pdf

-and-

https://rruff.geo.arizona.edu/doclib/am/vol75/AM75_295.pdf
(links normalized by FMF)

I don't know how or if the IMA uses substitutability as a criterion for lumping or splitting species. Does anyone know?

[Pete Richards]

[Matt_Zukowski]

I contacted John Hughes, the lead author on the two papers I referenced concerning substitutability, and he sent me a surprisingly quick and quite gracious reply.

In response to my question:
Is there a complete solid solution between Cl, F, and OH in the apatite subgroup in nature, or are natural Ca-PO4 apatites modal?

He wrote:
I believe that we do not know how there is a solid solution among the three end-members, if there is at all. I cannot explain complete solid solution along the F-Cl or OH-Cl joins except by invoking vacancies or symmetry breaking. I continue to investigate this with several colleagues, although one of the most difficult aspects of the research is synthesizing OH-free materials, and one must wonder if indeed there is an environment on Earth that would even allow crystallization of OH-free apatite.

In response to my question:
Why is the IMA a "splitter" (rather than "lumper") in the apatite subgroup nomenclature? To put it another way, why are there three species in a subgroup rather than a mineral called "apatite" with three varieties?

He wrote:
The IMA is nothing more than a group of mineralogists, and sometimes takes popular stances, and sometime unpopular. Like many mineral groups they "split" by chemistry rather than "lump" by structure type. They recently reversed some very unpopular decisions regarding apatite nomenclature, which relieved many in the community.

[Matt_Zukowski]

I also contacted John Rakovan, who has done a lot of research on apatite group minerals and a member of the IMA subcommittee on apatite group minerals, and his reply was:

A very good review of what is known about apatites is the MSA Reviews in Mineralogy and Geochemistry volume on Phosphates. The title is a little misleading because most of the volume is about apatite with a few chapters on other phosphate minerals. The exact nature of F-Cl-OH solid solutions is still an open question, although there is extensive mixing among the three. P. Piccoli & P. Candela in their chapter on igneous apatite show hundreds of data points regarding these substitutions.

The names fluorapatite, chlorapatite and hydroxylapatite date back about 150 years, long before the IMA existed and before there was any international effort to make a consistent mineralogical nomenclature. Because of this history and the tens of thousands of documents using these names, the recent IMA decision is to retrain them as is.